UNIX neophytes are often to be heard raving about how “everything is a file”. In UNIX, things are simple, after all, and this is to be celebrated, we are told.

What they really mean to say is that “every process accesses data via file descriptors using the open(), close(), read(), and write() system calls”, but neither does that roll off the tongue as easily nor sound like a reason for celebration. Now, this uniformity is all well and good from an API perspective, but if everything were indeed a file, then every file, directory, and network socket would be addressable by name system-wide (or perhaps in the same namespace if one is thinking in terms of Linux containers), and not just per-process file descriptor), and the celebrations would be back on.

This article is not about that, though.

In the current POSIX setup, directories are opened via opendir(), read by readdir(), and closed by closedir(). These three are library functions, not system calls (and perhaps this is the problem). Regular files are opened via open(), read from by read(), written to by write(), and closed by close().

But imagine if a given path on a system could be opened either as a file or as a directory depending on the desired usage. What user-level data representations would this enable? What would this prevent? What, if anything, would this added functionality break?

This would allow a normal-looking regular file such as some/where/file.dat to possess “sub-resources” (this is not an official term, just one I’ll use in this article, and they’ll appear as underlined italic for clarity) such as some/where/file.dat/index-001.txt, some/where/file.dat/summary.txt, or some/where/file.dat/results.txt. An entire directory tree could exist in the same places as these sub-resources given as examples.

Similarly, a directory such as some/where/amazing.app/ which looks like an application directory could be opened as a file and therefrom one could some metadata might be readable, such as a set of compatibility requirements, or application signatures.

In short then, under this scheme, all files may be opened as directories, and all directories may be opened as files. The former gives the ability to add multiple files of metadata to an existing directory of data, for example. The latter allows for applying metadata labels to a directory, among other possibilities.

references https://www.microsoft.com/en-us/research/uploads/prod/2011/10/WhatIsAFile.pdf